U.S. patent number 6,646,438 [Application Number 10/191,153] was granted by the patent office on 2003-11-11 for nmr data acquisition with multiple interecho spacing.
This patent grant is currently assigned to Baker Hughes Incorporated. Invention is credited to Martin Blanz, Thomas Kruspe, Peter Rottengatter, Holger F. Thern.
United States Patent |
6,646,438 |
Kruspe , et al. |
November 11, 2003 |
NMR data acquisition with multiple interecho spacing
Abstract
NMR data are acquired with variable spacing between refocusing
pulses, giving data with a variable interecho time TE. Under
certain conditions, diffusion effects can be neglected and data
acquired with a multiple TE spacing may be used to obtain a T.sub.2
distribution with increased resolution and reduced power
requirements. In gas reservoirs, the maximum TE may be determined
from diffusion considerations using a dual wait time pulse sequence
and this maximum TE is used to acquire data with dual TE. By proper
selection of TE, echos can be obtained with significantly reduced
ringing.
Inventors: |
Kruspe; Thomas (Wienhausen,
DE), Blanz; Martin (Celle, DE),
Rottengatter; Peter (Isernhagen, DE), Thern; Holger
F. (Hannover, DE) |
Assignee: |
Baker Hughes Incorporated
(Houston, TX)
|
Family
ID: |
25403097 |
Appl.
No.: |
10/191,153 |
Filed: |
July 9, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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894455 |
Jun 28, 2001 |
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Current U.S.
Class: |
324/303 |
Current CPC
Class: |
G01V
3/32 (20130101); G01R 33/50 (20130101) |
Current International
Class: |
G01R
33/50 (20060101); G01R 33/48 (20060101); G01V
3/18 (20060101); G01V 3/32 (20060101); G01R
33/44 (20060101); G01V 003/00 (); G01R
033/20 () |
Field of
Search: |
;324/303,300,306 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0489578 |
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Jun 1992 |
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EP |
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0967490 |
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Dec 1999 |
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EP |
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WO199701110 |
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Jan 1997 |
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WO |
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WO00/13044 |
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Mar 2000 |
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WO |
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WO01/42817 |
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Jun 2001 |
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WO |
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WO01/59484 |
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Aug 2001 |
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WO |
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Primary Examiner: Arana; Louis
Assistant Examiner: Fetzner; Tiffany A.
Attorney, Agent or Firm: Madan, Mossman & Sriram,
P.C.
Parent Case Text
CROSS-REFERENCES TO RELATED APPLICATIONS
This application is a continuation-in-part of U.S. patent
application Ser. No. 09/894,455 filed on Jun. 28, 2001.
Claims
What is claimed is:
1. A method for determining a parameter of interest of a volume of
earth formation surrounding a borehole with a Nuclear Magnetic
Resonance (NMR) logging tool conveyed in the borehole, the method
comprising: (a) using a magnet assembly on the NMR logging tool to
produce a static magnetic field in said volume of the formation;
(b) producing a radio frequency (RF) magnetic field in said volume
of the formation with an antenna on the logging tool, said RE
magnetic field having a component in a direction orthogonal to a
direction of the static field, the RF field including at least one
pulse sequence comprising a tipping pulse and at least four
refocusing pulses, each refocusing pulse producing at least one
corresponding spin echo signal; and (c) measuring with the logging
tool spin echo signals induced by the RE field in the formation;
wherein at least three successive later ones of said at least four
refocusing pulses of a single one of said at least one pulse
sequence have a time intervals greater than time intervals between
earlier successive refocusing pulses, and wherein at least one of
said at least four refocusing pulses produces more than one echo
signal.
2. The method of claim 1 further comprising selecting a time of
occurrence of at least one of said at least three refocusing pulses
to produce a spin echo signal substantially midway between a pair
of said at least three refocusing pulses.
3. The method of claim 1 further comprising selecting a time of
occurrence of at least one of said at least three refocusing pulses
to produce a spin echo signal later than midway between a pair of
said at least three refocusing pulses.
4. The method of claim 3 wherein said later spin echo signal shows
substantially less ringing artefacts than another echo between said
pair of refocusing pulses.
5. The method of claim 1 wherein the at least one pulse sequence
comprises a plurality of pulse sequences, one of said plurality of
pulse sequences having a wait time before the tipping pulse
different from a wait time before the tipping pulse of another of
said plurality of pulse sequences.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is related to methods for acquiring and processing
nuclear magnetic resonance (NMR) measurements for determination of
longitudinal and transverse relaxation times T.sub.1 and T.sub.2.
Specifically, the invention deals with methods for acquiring NMR
measurements using a modified CPMG sequences with a variable
interecho spacing.
2. Description of the Related Art
Nuclear magnetic resonance is used in the oil industry, among
others, and particularly in certain oil well logging tools. NMR
instruments may be used for determining, among other things, the
fractional volume of pore space and the fractional volume of mobile
fluid filling the pore space of earth formations. Methods of using
NMR measurements for determining the fractional volume of pore
space and the fractional volume of mobile fluids are described, for
example, in "Spin Echo Magnetic Resonance Logging: Porosity and
Free Fluid Index Determination," M. N. Miller et al., Society of
Petroleum Engineers paper no. 20561, Richardson, Tex., 1990.
Further description is provided in U.S. Pat. No. 5,585,720, of
Edwards, issued Dec. 17, 1996 and having the same assignee as the
present application, entitled "Signal Processing Method For
Multiexponentially Decaying Signals And Applications To Nuclear
Magnetic Resonance Well Logging Tools." The disclosure of the
Edwards patent is fully incorporated herein by reference.
Deriving accurate transverse relaxation time T.sub.2 relaxation
spectra from nuclear magnetic resonance (NMR) data from logging
subterranean formations, or from Cores from such formations, is
critical to determining total and effective porosities, irreducible
water saturations, and permeabilities of the formations. As
discussed in Prammer (U.S. Pat. No. 6,005,389), the total porosity
is the fractional volume of a rock that is occupied by fluids. The
total porosity (e.g. measured by a density tool) includes clay
bound water that typically has extremely short relaxation times,
moveable water and hydrocarbons that have long relaxation times,
and capillary bound water that has intermediate relaxation times.
The effective porosity is defined as that portion of the pore
volume containing fluids that are moveable, i.e., the total
porosity minus the clay bound water. Accurate spectra are also
essential to estimate T.sub.2 cutoff values and to obtain
coefficients for the film model or Spectral Bulk Volume Irreducible
(SBVI) model. Effective porosities are typically summations of
partial porosities; however, distortion of partial porosity
distributions has been commonly observed for a variety of reasons.
These reasons include poor signal-to-noise ratio (SNR), and poor
resolution in the time domain of the NMR data.
U.S. Pat. No. 6,069,477 to Chen et al having the same assignee as
the present application discusses the constituents of a fluid
saturated rock and various porosities of interest. Referring to
FIG. 1, the solid portion of the rock is made up of two components,
the rock matrix and dry clay. The total porosity as measured by a
density logging tool is the difference between the total volume and
the solid portion. The total porosity includes clay-bound water,
capillary bound water, movable water and hydrocarbons. The
effective porosity, a quantity of interest to production engineers
is the sum of the last three components and does not include the
clay bound water.
The most common NMR log acquisition and core measurement method
employs T.sub.2 measurements using CPMG (Carr, Purcell, Meiboom,
and Gill) sequence, as taught by Meiboom and Gill in "Modified
Spin-Echo Method for Measuring Nuclear Relaxation Time," Rev. Sci.
Instrum. 1958, 29, pp. 688-691. In this method, the echo data in
any given echo train are collected at a fixed time interval, the
interecho time (TE). Usually, a few hundred to a few thousand echos
are acquired to sample relaxation decay. However, for determination
of CBW, echo sequences of as few as ten echos have been used.
Interecho time (TE), is one of the most important, controllable
experimental parameters for CPMG measurements and can affect data
interpretation. In logging operations using the MRIL.RTM. tool
(made by Numar Corp.), TEs of 0.6 and 1.2 milliseconds (ms) are
typically used to manipulate the relaxation decay data to include
or exclude clay bound water (CBW) porosity.
Interpretation of NMR core or log data is often started by
inverting the time-domain CPMG echo decay into a T.sub.2 parameter
domain distribution. In general, the T.sub.2 of fluids in porous
rocks depends on the pore-size distribution and the type and number
of fluids saturating the pore system. Because of the heterogeneous
nature of porous media, T.sub.2 decays exhibit a multiexponential
behavior. The basic equation describing the transverse relaxation
of magnetization in fluid saturated porous media is ##EQU1##
where M is magnetization, and effects of diffusion in the presence
of a magnetic field gradient have not been taken into
consideration. Eq.(1) is based on the assumption that diffusion
effects may be ignored. In a gradient magnetic field, diffusion
causes atoms to move from their original positions to new ones
which also causes these atoms to acquire different phase shifts
compared to atoms that did not move. This contributes to a faster
rate of relaxation.
The effect of field gradients is given by an equation of the form
##EQU2##
where the first two terms on the right hand side are related to
bulk relaxation and surface relaxation while the third term is
related to the field gradient G by an equation of the form
##EQU3##
where TE is the interecho spacing, C is a constant and D is the
diffusivity of the fluid.
In CPMG measurements, the magnetization decay is recorded (sampled)
at a fixed period, TE; thus, a finite number of echos are obtained
at equally spaced time intervals, t=n TE, where n is the index for
the n-th echo. This may be denoted by ##EQU4##
A problem associated with conventional CPMG sequences is that the
resolvability of the T.sub.2 spectrum is not uniform. Short T.sub.2
s are poorly resolved as only a few data points are affected by
these components. Long T.sub.2 s, on the other hand, are
oversampled. In addition, due to limitations on availability of
power, the number of pulses is limited: this has the undesirable
effect of leading to poor resolution of short T.sub.2 components
because measurements have to be made over long time to resolve the
slowly relaxing components. The actual selection of TE and number
of pulses involves a tradeoff governed by the power availability
and the desire for rapid acquisition to keep down rig costs.
As discussed in U.S. Pat. No. 6,069,477 to Chen et al, the contents
of which are fully incorporated herein by reference, the effects of
noise, sampling rate, and the ill-conditioning of inversion and
regularization are to smear (broaden) the estimated T.sub.2
distribution. In addition, because of the non-orthogonality of
multi-exponential signals, CBW signals could be shifted to higher
T.sub.2 regions if the T.sub.2 fitting region is limited or if the
regularization is excessive. This distortion is not easily
rectified; even adding more bins with short T.sub.2 does not reduce
the distortion of the T.sub.2 spectra.
Chen et al teaches the use of CPMG sequences with two different
values of TE (0.6 ms and 1.2 ms). A time domain correction is used
to filter out the contribution of the fast relaxing T.sub.2
components in the TE=1.2 ms echo train. High S/N echo data with
sampling time TE=0.6 ms are used to obtain the CBW T.sub.2
distribution. These data are then used to reconstruct CBW
contributions to the time domain early echos of the conventional
effective porosity echo data (TE=1.2 ms). The CBW signal is then
subtracted from the original echos, and the effective porosity
distribution is estimated from the reconstructed echo train. Other
methods can be applied to jointly process echo train data with
different TEs (e.g., Dunn et al., 1998, "A method for inverting NMR
data sets with different signal to noise ratios" paper JJ, in 39th
annual logging symposium transactions SPWLA).
The use of two different CPMG sequences with different values of TE
may still lead to erroneous results in measurement-while-drilling
applications. The reason is that due to tool motion, the sensitive
volume may be different for the first and second CPMG sequence. The
region of investigation for an NMR logging tool is defined by the
region in the formation wherein the Larmor frequency of nuclear
spins matches the RF frequency of the tipping pulse. Subsequent
refocusing pulses in a CPMG sequence will produce spin-echo signals
from this region. When the tool is in motion, as in a MWD logging
tool, the region of investigation may be different for successive
CPMG sequences, so that the spin echos for a second CPMG sequence
may not come from the same region as the spin echos for a first
CPMG sequence. It would be desirable to have a method of obtaining
an NMR spectrum of a medium with a resolution that is more uniform
over the range of spectral values and that is not sensitive to tool
motion, particularly in MWD applications. Such a method should
preferably also have reduced power consumption. The present
invention satisfies this need.
SUMMARY OF THE INVENTION
The present invention is a method of acquiring NMR spin echo
signals using pulse sequences having more than one interecho
spacing. This makes it possible to acquire relaxation spectra with
high resolution of rapidly relaxing components and reduced power
requirements over the slowly relaxing portions of relaxation
spectra. Multiple TE data may be acquired when different types of
fluids are present in the formation to resolve the fast decaying
components as well as the slow decaying components. In a gradient
field the diffusion effect has to be considered for selection of
proper TE values.
In a preferred embodiment, the method is used with a zero gradient
magnetic field configuration. This reduces the effects of diffusion
on the echo signals. Power requirements may be further reduced by
using refocusing pulses with an angle of less than 180.degree..
Signal to noise ratio may be further improved by using a plurality
of pulse sequences and stacking the resulting signals. By proper
selection of the variable TE sequences a desired resolution may be
obtained for all expected components (short, medium, and long)
while reducing the required time and the required power. This is
particularly important in the resolution of short T.sub.2
components.
When used for measurement-while-drilling, optional embodiments of
the invention use motion sensors on the drilling assembly and the
timing of the pulses in the pulse sequences is based in part on the
output of the motion sensors. Optionally, predictive filtering of
the motion signals may be used to further improve the signal to
noise ratio.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the different constituents of a fluid filled rock.
FIG. 2 (PRIOR ART) is a schematic cross-section of a NMR
measurement-while-drilling tool in a borehole
FIG. 3 shows an example of a pulse sequence with non-uniform
interecho times.
FIG. 3a shows an alternative example of a pulse sequence with
non-uniform interecho times.
FIG. 4 shows the sampling of a decay curve using 3 different
interecho times.
FIG. 5 shows echos created by the transition from short interecho
time to a long one.
DETAILED DESCRIPTION OF THE INVENTION
A suitable device for use of the present invention, is disclosed in
U.S. Pat. No. 6,215,304 to Slade, the contents of which are fully
incorporated herein by reference. It should be noted that the
device taught by Slade is for exemplary purposes only, and the
method of the present invention may be used with many other NMR
logging devices, and may be used for wireline as well as MWD
applications. Examples of such devices are given in U.S. Pat. No.
5,557,201 to Kleinberg, U.S. Pat. No.5,280,243 to Miller, U.S. Pat.
No.5,055,787 to Kleinberg, and U.S. Pat. No.5,698979 to
Taicher.
Referring now to FIG. 2, the tool has a drill bit 7 at one end, a
sensor section 2 behind the drill head, and electronics 1. The
sensor section 2 comprises a magnetic field generating assembly for
generating a B.sub.0 magnetic field (which is substantially time
invariant over the duration of a measurement), and an RF system for
transmitting and receiving RF magnetic pulses and echos. The
magnetic field generating assembly F comprises a pair of axially
spaced main magnets 3,4 having opposed pole orientations (i.e. with
like magnetic poles facing each other), and three soft magnetic
members 9,10 axially arranged between the magnets 3,4. The soft
magnetic members can be distinguished over "hard" magnetic material
by the shape of the BH curve(hysteresis). Permanent magnets are
characterized by hysteresis curve that encloses a large area while
in an ideal soft magnet there is no hysteresis at all. Real soft
magnetic material do show a small hysteresis. The slope of the BH
curve is identical to the magnetic field constant (or absolute
permeability) .mu..sub.0 times the (relative) permeability
.mu..sub.T. Usual soft magnetic materials are NiFe alloys, soft
ferrites, polymer bound iron powder or amorphous metals. The
permeability typically ranges from about 10 (polymer bound iron
powder) to 100000 (amorphous metal). Where RF magnetic field needs
to penetrate the soft magnet, electrically non-conductive or
layered conductive material need to be used. Examples for
non-conductive materials are ferrites or polymer bound iron powder.
The RF system comprises a set of RF transmit antenna and RF receive
antenna coil windings 5 arranged as a central "field forming"
solenoid group 13 and a pair of outer "coupling control" solenoid
groups 14.
The tool has a mud pipe 60 with a clear central bore 6 and a number
of exit apertures 61-64 to carry drilling mud to the bit 7, and the
main body of the tool is provided by a drill collar 8. Drilling mud
is pumped down the mud pipe 60 by a pump 21 returning around the
tool and the entire tool is rotated by a drive 20. Coded tubing or
a drillstring may be used for coupling the drive to the downhole
assembly.
The drill collar 8 provides a recess 70 for RF transmit antenna and
RF receive antenna coil windings 5. Gaps in the pockets between the
soft magnetic members are filled with non-conducting material 31,
35 (e.g.: ceramic or high temperature plastic) and the RF coils 13,
14 are then wound over the soft magnetic members 9, 10. The soft
magnetic material 9, 10 and RF coil assembly 13, 14 are pressure
impregnated with suitable high temperature, low viscosity epoxy
resin (not shown) to harden the system against the effects of
vibration, seal against drilling fluid at well pressure, and reduce
the possibility of magnetoacoustic oscillations. The RF coils 13,
14 are then covered with wear plates 11 typically ceramic or other
durable non-conducting material to protect them from the rock
chippings flowing upwards past the tool in the borehole mud.
Because of the opposed magnet configuration, the device of Slade
has an axisymmetric magnetic field and region of investigation 12
that is unaffected by tool rotation. Use of the soft magnetic
material results in a region of investigation that is close to the
borehole. This is not a major problem on a MWD tool because there
is little invasion of the formation by borehole drilling fluids
prior to the logging. The region of investigation is a shell with a
radial thickness of about 20 mm and an axial length of about 50 mm.
The gradient within the region of investigation is less than 2.7
G/cm. It is to be noted that these values are for the Slade device
and, as noted above, the method of the present invention may also
be used with other suitable NMR devices.
Turning now to FIG. 3, an exemplary pulse sequence according to the
method of the present invention is disclosed. Following a wait time
of T.sub.w, the nuclear spins in the region of investigation will
be aligned substantially parallel to the direction of the static
magnetic field. At the end of the wait time, a 90.degree. tipping
pulse 101 is applied. The effect of this is to tip the nuclear
spins into a plane orthogonal to the direction of the static field.
At the end of the tipping pulse, the nuclear spins start precessing
in this orthogonal plane and also dephasing with a time constant
T.sub.2 * due mainly to B0 field inhomogeneities in the static
magnetic field B.sub.0. Using the CPMG pulse sequence this T.sub.2
* dephasing effect is cancelled and the resulting spin echo train
amplitudes decay with time constant T.sub.2. Application of a
refocusing pulse 103a reverses the direction of precession so that
at a time 105a a spin echo signal is produced that may be detected
by the antenna. Subsequent refocusing pulses 103b, 103c . . . 103f,
103g, 103h will produce additional spin echo signals 105b, 105c, .
. . , 105g. At the end of the tipping pulse, the nuclear spins
start precessing in this orthogonal plane and also dephasing with a
time constant T.sub.2 * due mainly to B0 field inhomogeneities in
the static magnetic field B.sub.0. Using the CPMG pulse sequence
this T.sub.2 * dephasing effect is cancelled and the resulting spin
echo train amplitudes decay with time constant T.sub.2. Application
of a refocusing pulse 103a reverses the direction of precession so
that at a time 105a a spin echo signal is produced that may be
detected by the antenna. Subsequent refocusing pulses 103b, 103c .
. . 103f, 103g, 103h will produce additional spin echo signals
105b, 105c, . . . , 105g.
A novel feature of the present invention is that the spacing
between the refocusing pulses need not be uniform. In the example
shown in FIG. 3, the time interval between the refocusing pulses
103a, 103b and 103c is less than the time interval between later
pulses 103e, 103f, 103g,. If additional intervals are used, the
spacing may be even larger.
Turning now to FIG. 4, the spin-echo signals obtained by the pulse
sequence such as that shown in FIG. 3 are shown. The abscissa 200
is time and the ordinate 202 is the echo amplitude A first
plurality n.sub.1, of echos 201a, 201b, 201c. 201d, 201e are
obtained with a spacing T.sub.E1, these are followed by a second
plurality n.sub.2 of echos 211a 411b . . . 211k obtained with a
spacing T.sub.E2 and may be followed by additional pluralities
n.sub.3 of echos 221a 221b, 221c, obtained with a spacing T.sub.E3
etc. As can be seen, using the sequence of FIG. 3 results in the
early part of the sequence (where the amplitudes decay rapidly,
corresponding to small relaxation times) being more closely sampled
than the latter part of the sequence (where the amplitude decay is
slow, corresponding to long relaxation times). In order to generate
an NMR-echo decay with the timing in FIG. 4, which has three
distinct interecho times T.sub.E1, T.sub.E2 and T.sub.E3 it is
necessary to increase the time between refocusing pulses T.sub.P at
each transition between two T.sub.E s in two steps. This is shown
in FIG. 3; between the times T.sub.P1 and T.sub.P2, a transition
time T.sub.PT =(T.sub.P1 +T.sub.P2)/2 is inserted. Otherwise the
spin echo after a transition would not form midway between two
refocusing pulses.
The sparse sampling of the later portions of the echo train also
means that the overall duty cycle on the power source is reduced.
This makes it possible to sample large relaxation components
without unduly burdening the power supply.
Those versed in the art would recognize that one effect of the
variation in the interecho spacing may be a variation in T.sub.2
due to diffusion effects. The diffusion effects are proportional to
the square of the field gradient and the interecho spacing and
directly proportional to the diffusivity of the fluid. This effect
is minimized by the magnet design in the Slade device; with a field
gradient of less than 3.0 G/cm, the effect of diffusion of bulk
water (with T.sub.2B =2.5 s) at room temperature is negligible for
TE up to 2 ms, while at 150.degree. C., the effect is small for TE
up to 0.8 ms. For water in a porous medium the TE values will
increase depending on the pore size. In general, diffusion effects
are small for water, heavy oil, medium oil and light oil but may
not be negligible for gas or very light liquids and condensate.
U.S. Pat. No. 6,331,775 of Thern et al, the contents of which are
incorporated herein by reference, teaches the use of a dual wait
time echo data for determination of gas saturation in the
formation. The wait times are selected to substantially polarize
the liquid phase but to produce substantially different
polarization of the gas phase. In one embodiment of the invention,
dual wait time data are acquired and, on the basis of evaluation of
the results in a downhole processor, an estimate of the gas
saturation is made. Based on this estimate, a maximum allowable TE
is determined and subsequent data are acquired with a dual TE pulse
sequence wherein the first few echos (up to 10) are acquired with a
short TE (e.g. of 0.5 ms) , enhancing the resolution of short
T.sub.2 s and the remaining echos are acquired with a longer but
fixed TE (e.g., 2-4 ms).
U.S. Pat. No. 6,005,389 to Prammer teaches the use of a plurality
of pulse sequences with short TE s (approximately 0.5 ms) and
summing the echo trains to improve the signal to noise ratio of the
earliest pulse echos. Using this summed data, Prammer teaches the
determination of rapidly relaxing components of the T.sub.2
distribution. In one embodiment of the present invention, a
plurality of pulse trains with variable TE are acquired and by
stacking the signals, the entire T.sub.2 distribution may be
obtained. This makes it possible to determine the total porosity,
clay bound water and effective porosity. In an optional embodiment
of the present invention, the variable TE method of the present
invention may also be used in conjunction with a plurality of wait
times T.sub.w.
In one embodiment of the invention, the choice of TE is made during
the drilling process. Dual or multiple TW data are acquired during
the process of drilling using a fixed TE using prior art methods.
Evaluation of these data makes it possible to estimate the T.sub.1
and the T.sub.2 distribution in real time using a downhole
processor. The estimation of the T.sub.2 distribution may be done
directly or indirectly using a fixed relation between T.sub.1 and
T.sub.2. Knowing the maximum value of T.sub.2 for the formation,
the maximum TE is estimated using eqs.(2) and (3). The field
gradient is a known quantity and the diffusivity is estimated from
knowledge of the fluid type, the rock type and the porosity. The
fluid type, rock type and porosity are determined downhole from
other logs such as density, gamma ray and resistivity logs.
A particular advantage of the present pulse sequence is that the
same 90.degree. tipping pulse is used for obtaining data at a
single frequency with a variable interecho spacing. This avoids
problems of different regions of excitation that may be caused by
transversal tool motion when the multiple frequency method of Chen
et al is used. However, the use of a varying TE results in a more
complicated sequence of echos. This is illustrated in FIG. 5
wherein after an initial tipping 90.degree. pulse 301 there is
180.degree. refocusing pulse 303 delayed by time .tau..sub.1 and a
second refocusing pulse 305 at a time .tau..sub.2 after the first
refocusing pulse. The pulse echo 311 results from the tipping pulse
301 and the first refocusing pulse 303. Following the second
refocusing pulse 305, four echos are produced at times 2.tau..sub.1
+.tau..sub.2, 2.tau..sub.2, .tau..sub.1 +2.tau..sub.2, and
2.tau..sub.1 +2.tau..sub.2. In FIG. 5 these echos are labeled 313,
315, 317 and 319 respectively. The earliest of these echos 313 is
produced by all three RF pulses 301, 303 and 305 and is known in
the NMR literature as the stimulated echo. The second echo 315
after the refocusing pulse 305 is the so-called secondary echo and
also produced by all three pulses. The further echos 317 and 319
are two-pulse echos caused by the first or second pulse together
with the third pulse. Of all these NMR signals, echo 315 would be
typically the most useful, but the other echos may be acquired as
well. Details of the three-pulse echo generation can be found in
chapter 2.2, FIG. 2.2 of R. Kimmich "NMR--Tomography, Diffusometry,
Relaxometry", ISBN 3-540-61822-8, Springer-Verlag Berlin Heidelberg
N.Y.
The echos 313, 317 and 319 would only be present where the
refocusing pulse is not 180.degree.. For a NMR sample of limited
size, for which the frequency band width of the refocusing pulse is
much greater than the bandwidth of NMR frequencies of the sample
and the refocusing pulses are 180.degree., the echos would be as
shown in FIG. 4. In the practice of oil well logging such an ideal
situation is virtually impossible so that there would be echos in
addition to those shown in FIG. 4. These additional echos are
indicated in FIG. 3 as 107d, 107e, 107f . . . As a result of this,
the centered echos (105d, 105e, 105f) after the change of TE are of
lower amplitude than before the change. Before the evaluation of
the relaxation measurement these later echos need to be scaled by
correction factors similar to the well known "stimulated echo
correction factors".
The aim of the pulse sequence in FIG. 3, which uses a transitional
inter pulse time T.sub.PT at the change from TE.sub.1 to TE.sub.2,
is the generation of echos midway between two pulses. This,
however, is not essential. FIG. 3a shows an example of a pulse
sequence that changes the inter pulse time from T.sub.P1 to
T.sub.P2 without a transitional time step. Two echos between each
two pulses are formed after the T.sub.P step, i.e. echo pairs
115d/115e, 115f/115g, 115h/115i etc. Both echos of each pair may be
acquired, thereby increasing the total acquired NMR signal. These
later echos are reduced in amplitude with respect to the earlier
echos 115a to 115c and need to be corrected with correction factors
before evaluating the T.sub.2 relaxation time distribution. If at a
later time the interpulse spacing is increased again, even more
echos may be generated and can be acquired if desired.
FIG. 3a illustrates also another interesting aspect of the multiple
TE measurement. The unequal spacing of pulses can be used to
generate echos that appear not midway between pulses but later,
closer to the later pulse. The later echo may be of good quality
while the earlier echo or even an echo in the center between pulses
may be distorted due to ringing (e.g. acoustic or electronic
ringing). The unequal RF pulse spacing, therefore, allows the
acquisition of echos in situations of short TE where conventional
centered CPMG echos may be distorted excessively by ringing.
Those versed in the art would recognize that in a conventional CPMG
sequence, each of the refocusing pulses has a duration and
amplitude designed to change the angle by 180.degree.. One
embodiment of the invention uses refocusing pulses with a smaller
tipping angle. Such a pulse sequence has been disclosed in U.S.
Pat. No. 6,163,153 to Reiderman et al, the contents of which are
fully incorporated herein by reference.
One embodiment of the present invention uses the teachings of
co-pending U.S. patent application Ser. No. 09/778,205 of Hawkes et
al, the contents of which are incorporated herein by reference.
Hawkes et al teaches the use of motion triggered pulsing for NMR
measurements. The motion of the tool is measured by suitable motion
sensors, such as accelerometers, magnetometers or gyroscopes or
combinations thereof. These sensors may be placed at any suitable
location on the drilling tool in the proximity of the magnet and
coil arrangement. The wait period in a pulse sequence may be
extended slightly without affecting the data quality and this
feature may be used to delay the application of the tipping pulse
until a suitable state of tool motion is achieved. The trigger may
be obtained by monitoring the motion sensor signals. Suitable
states for triggering are instantaneous moments when the tool is
stationary, or if the motion has a strong periodic component, then
subsequent pulse sequences may be triggered to synchronize with
this motion. Such motion triggered pulsing will improve the NMR
spin-echo formation.
It is common practice in NMR measurements to make multiple
measurements of pulse echos and to average the measurements. The
Hawkes application also teaches the use of a predictive filter
using the output of the motion sensors to predict the motion of the
drillstring. In the case of motion containing one or more periodic
components, using the output of the predictive filter, data
acquisitions can be triggered when the NMR tool is in approximately
the same position, stationary or in the same state of motion, and
the data averaged to improve the signal to noise ratio.
An optional embodiment of the invention that may be used with any
of the pulse sequences described above is used to reduce the effect
of ringing. In the so-called phase alternated pulse (PAP) sequence,
the successive CPMG or modified CPMG sequences are acquired with
alternating phases of the tipping pulse. Summing the echos from
such a PAP sequence reduces the effects of ringing.
While the foregoing disclosure is directed to the preferred
embodiments of the invention, various modifications will be
apparent to those skilled in the art. It is intended that all
variations within the scope and spirit of the appended claims be
embraced by the foregoing disclosure.
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